Loopy

from Simon Tatham's Portable Puzzle Collection

Form a single closed loop out of the grid edges, in such a way that
every numbered square has exactly that many of its edges included in
the loop.

Click on a grid edge to mark it as part of the loop (black), and
again to return to marking it as undecided (yellow). Right-click on
a grid edge to mark it as definitely not part of the loop (faint
grey), and again to mark it as undecided again.

When you have mastered the square grid, look in the Type menu for
many other types of tiling!